The invention relates to a filter unit and apparatus for desludging salt baths, which is at least partially flooded by the salt bath and is in working connection with the upper end of a riser tube, while the bath fluid that is to be desludged is forced by means of a compressed-air stream through the riser toward the filter unit.
Salt baths are widely used, for example for the hardening or nitriding of metal materials, especially iron materials. Such salt baths consist predominantly of carbonates and cyanates of the alkali metals, and to some extent alkali cyanides are also contained in them. Such molten salts are normally operated at a temperature of 560.degree. to 600.degree. C.
During the treatment of components made from ferrous materials these baths form sludge in varying degrees, due to the fact that adherent steel particles, such as turning chips or grinding dusts, collect on the bottom of the treatment vessel or remain suspended in finely divided form. Likewise, the treatment of scaly or unworked surfaces (castings or forgings) leads to contamination of the bath. Lastly, sludge content is also formed by the fact that loading aids, such as baskets or frames, flake off nitride particles after many uses, which remain in the bath. The "sludge" thus formed consists mainly of iron nitride and iron oxide.
Other salt baths, such as quenching baths on a hydroxide basis, become increasingly thick due to the formation of insoluble carbonates, causing the melting point of the baths to increase and the quenching effect to diminish. Therefore salt baths of this kind must also be cleaned up.
Since the sludge content of the nitriding bath has a great influence on the repeatability of the results of the nitridation, a constant cleaning of the bath is very important. In practice, a variety of desludging apparatus are used for the purpose. All of the former systems have it in common that their operation has to be interrupted for desludging, i.e., the desludging must be performed on an unloaded bath.
DE-OS 29 11 222 discloses an apparatus for the continuous desludging of salt baths, which consists of a filter unit, a pump immersed in the salt bath, and a riser tube. The filter unit is in this case disposed outside of the salt bath.
Also known is an apparatus for the discontinuous desludging of salt baths serving for the treatment of metals, consisting of a pumping tube, an air feed line and a filter unit (DE 38 36 939) in which glass wool or woven iron screens with a mesh size of 0.05 to 0.5 mm are used as filter materials.
Woven screens of iron wire, however, have the disadvantage that the iron material is severely attacked--embrittled for example--by the salt bath itself and also by the substances contained in the filter sludge, so that after a few hours of operation the screen tears or breaks open or develops holes, with the result that the retained filter sludge pours through into the cleaned bath and abruptly renders it unusable.
Also known (DAS 27 31 167) is the use of inorganic molded materials, especially glass wool or rock wool, for the cleaning of molten salt baths, especially for the removal of harmful suspended substances and settled sludge in baths for the structural treatment of ferrous materials. These glass wool or rock wool filters, however, have the disadvantage among others that they are hard to dispose of when contaminated, so that they are impractical for reasons of cost.
Lastly, a filtering means with an electrogalvanically built-up filter medium is known (DGM 92 00 734.1), wherein the filter medium is made from nickel or a nickel alloy and has a thickness of 0.1 to 0.2 mm. This known filter medium, however, in the form of a pressure screening fabric, is suitable only as a multi-stage separating system, a band filter for example, which is connected to the output of hydrocyclones.